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San Francisco resident Frank Therre learned this the hard way. And as chief operating officer for a Bay Area software firm, he isn't exactly unsophisticated when it comes to technology.

"I was very surprised to learn that cell phone numbers are recycled," Therre told me. "You never know who had the number before. Some pretty serious situations can come out of this."

In his case, he said his company signed up with Sprint in November to create a back-up line for customer support.

Therre said he plugged in the new phone overnight and was greeted the next morning with a text message reading: "Cindy wants to meet you. If you want to meet Cindy. ..."

It invited Therre to send a text message to Cindy. He didn't bother and didn't give the matter another thought.

Therre said the cell phone was given to a company technician responsible for fielding calls from customers. That's where the problems started.

The technician's fiancee happened to look at the phone one day. She couldn't help but notice dozens of text messages from a seemingly wide assortment of women eager to meet her man.

"This got him into quite a bit of trouble," Therre said.

Therre then checked his company's Sprint bill. He was startled to find a $9.90 charge for no fewer than 99 text messages being received, plus an "additional charge" of $16.50 related to the service.

Therre said he called Sprint and was told by a service rep that he must have signed up for a premium service. He said Sprint's records showed that the text messages were being billed by a company called SMS.ac.

Therre replied that he never signed up for any service from this company. But he said the rep was adamant: It was Therre's problem.

So he contacted SMS.ac, which turns out to be a San Diego firm that creates networks of people who send text messages to one another.

The service also lets users purchase products and services via their phones, with the charges appearing on their phone bills.

Therre said SMS.ac confirmed that his cell phone number had been signed up for the service. But the start date was June 2005 -- five months before Therre got the phone -- and the service was registered to someone named Morales.

"I don't know anyone named Morales," Therre said.

He told me that SMS.ac promptly canceled the account and explained that situations like this sometimes happen when a cell phone number is switched to a new customer.

An SMS.ac rep said it's the responsibility of the wireless carrier to ensure that all outstanding obligations are canceled before a phone number is reused.

Greg Wilfahrt, executive vice president of SMS.ac, confirmed to me that his company's recurring charges can indeed be carried over to noncustomers who receive a recycled number.

"It's something we're conscious of," he said. "It can happen."

Caroline Semerdjian, a Sprint spokeswoman, said the carrier makes it a habit to wipe clean all links to a former customer's phone number.

As such, she said she couldn't imagine how Therre's line was billed for a previous customer's service.

But Semerdjian acknowledged that Sprint, like most wireless carriers, does recycle cell phone numbers. She said a number might be handed out to a new customer 30 days after an old account is closed.

"There aren't enough new numbers available for everyone who wants one," Semerdjian explained.

A spokeswoman for Verizon Wireless said her company also recycles phone numbers after just 30 days.

Cingular waits at least 90 days before recycling numbers. A spokeswoman said the company uses special software to block any bills related to a former customer.

A T-Mobile spokeswoman said her company also has a 90-day waiting period before numbers are recycled.

Michael Shames, executive director of the Utility Consumers' Action Network in San Diego, said he's received numerous complaints from people in recent months about wireless customers getting calls for a line's former owner.

"Each call can eat up your minutes, so there's a real impact," he said. "Why can't all the wireless providers wait at least 90 days, or even 180 days? That would go a long way toward preventing problems for new customers."

Therre's case, Shames added, only underlines how important it is for people to closely inspect their bills after signing up for a wireless service.

The growing practice (especially among teens) of downloading ring tones from services with monthly fees shows how easy it is for a cell phone number to be on the hook.

Therre told me he called Sprint back after resolving things with SMS.ac, and this time a rep acknowledged that numbers get recycled and that problems can occasionally occur.

"He said they try to do their best to clear accounts but made a point of saying that they can't be held responsible," Therre said.

I'm guessing there might be a lawmaker or two reading this who think otherwise.

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